Bleeding edge Linux distros (such as Arch Linux or Fedora) have switched some time ago to glibc 2.28. It has broken building of bison and m4 host packages. It can be fixed by using these patches (the same problem was found in Fedora and OpenWRT): https://github.com/openwrt/openwrt/commit/8ae84899865ea14ce9d55cffa5de5ddd7167c680 https://github.com/openwrt/openwrt/blob/e5d47f32131849a69a9267de51a30d6be1f0d0ac/tools/bison/patches/110-glibc-change-work-around.patch
Separate bugs for bison and m4 would have been better. Indeed, m4 is already fixed: https://git.buildroot.org/buildroot/commit/?id=c48f8a64626c60bd1b46804b7cf1a699ff53cdf3, but Bison is not fixed yet.
Bison has been fixed by https://git.buildroot.org/buildroot/commit/package/bison?id=373ac58b82d2430b745e90c7c8b39e319949882a, so the bug is fixed.